This invention relates to off-set lithography and more particularly to apparatus for adjusting the position of the printed image produced by an off-set lithographic printer, the apparatus acting to change the relative disposition of the image carrying plate segment in a simple and precise manner.
In the art of lithographic printing an image carrying plate, generally a thin metallic plate, has image areas including an affinity for ink while repelling moisture, and non-image areas which retain moisture and do not accept ink. In an off-set lithographic printer, this image plate is mounted on a segment of a rotational cylindrical drum, while another segment of the drum carries a pressure member, both of which act in seriatim in conjunction with an impression receiving and transferring drum rotatably driven in timed relationship with the image carrying drum. The segment of the cylindrical drum which carries the image plate is known in the art as the plate segment while the segment which carries the pressure member is known as the impression segment, both of which are mounted on a rotatably driven skeletal cylindrical frame. The impression receiving and transferring drum is known in the art as the blanket cylinder since its surface is covered by a mat or blanket generally formed from rubber or the like. For each revolution of the image plate carrying cylinder, the blanket cylinder rotates twice. During the first rotation of the blanket cylinder, the blanket contacts the plate segment and receives the ink from the image plate so that the image is transferred onto the blanket. During the second rotation of the blanket cylinder, a sheet of paper is fed to the nip between the blanket and the impression segment of the image plate carrying cylinder. The pressure exerted by the impression segment forces the paper against the blanket and the image thereon is transferred or printed onto the paper. During the rotation of the image plate carrying cylinder after the image plate has transferred its image onto the blanket cylinder it is moistened so that water is transferred to the non-image areas of the plate and thereafter contacted by the ink applying rollers, the ink being transferred only to the areas not retaining water.
It is thus clear that the disposition of the plate segment, and thus the image plate, is determinative of the location of the image on the blanket cylinder and thus the position of the image printed on the paper. This position of the image on a sheet of paper can be critical in many printing operations. This is especially critical where multi-color off-set printing is performed. For example, in multi-color off-set printing an image is printed during a first run with one color and thereafter the same image is subsequently printed again with other individual colors in separate runs, different areas of the image being made receptive to ink for the different colors. The final product will thereby have the different colors disposed in the proper locations if the image plate has been located identically on the image plate carrying cylinder for each color. Since the image plate is clamped to the plate segment by a pair of spaced clamping members, identical positioning of the image plate relative to the plate segment is difficult, if not impossible to accomplish for each color change. Consequently, it is more accurate to mount the image plate on the plate segment as accurately as possible and thereafter correct for misalignment by changing the disposition of the plate segment relative to the skeletal frame of the cylinder.
Various approaches have been made in the prior art to provide accurate adjustment of the plate segment relative to the frame of the cylinder. For example, one method requires the locking of the plate segment against movement relative to the frame of the machine by exerting a pin into a hole in the edge of the plate segment, thereafter loosening the screws which secure the plate segment to the cylinder skeletal frame, and thereafter holding the pin in the hole and turning the machine by hand until the cylinder has been moved the desired amount relative to the plate segment. The securing screws are then tightened and the pin removed. If one inadvertently forgets to remove the pin from the hole after adjustment, great damage may occur to the machine elements when the machine is started. Moreover, even though a tapered pin is utilized, as the pin receiving hole wears, proper locking of the plate segment and adjustment thereof becomes more difficult as the pin tends to slip. If the plate segment is not locked properly, precise adjustment is not attained.